1. Field of the Invention
This invention relates to a tough board of thermoplastic resin foam having a prominent mechanical, particularly compressive and bending strength and to a method of manufacturing the same.
2. Description of the Prior Art
In recent years, plastics foamed articles are utilized in large quantities over a wide field due to excellent properties of heat insulation, shock absorption and buoyancy. However, one of the drawbacks of such plastics foamed articles is low mechanical strength. Therefore, demand has been made for the development of plastics foamed articles characterized by high mechanical strength.
Some foam manufactured from thermoplastic resin by conventional methods is a soft mass generally formed of fine cells having a density of about 0.03 g/cm.sup.3, and presents prominent heat-insulating and shock-absorbing properties, but has extremely small mechanical strength.
By way of contrast, U.S. Pat. No. 3,523,988 discloses a method of manufacturing a foam of great mechanical strength in order to attain improvements in the aforesaid drawbacks accompanying foams of the prior art. According to such patented method the die includes an expansion chamber to reduce pressure therein. In a die slit next to said expansion chamber, there is formed a mixture of polyolefin, volatile foaming agent and nucleating agent. Compression is applied to collapse the cells on the surfaces of the foam so as to produce a hard skin. The foam is fully expanded in said expansion chamber under a certain pressure. Such method differs from this present invention in that the foam is expanded while it is subjected to a shearing stress in a confined zone. Accordingly, the foam obtained by the U.S. patent method consists of cells less than 5 mm in diameter and has a density of 0.16 g/cm.sup.3 at the lowest, and a thickness of less than 10 mm.
The present inventors previously provided a method of manufacturing plastics foamed articles having prominent mechanical strength by means of an extrusion process. As disclosed in U.S. Pat. No. 3,863,000, a mixture of plastics material and foaming agent is extruded through an extruder in the form of a board. The upper and lower surfaces of the board are quenched prior to foaming to form skin layers thereon. Thereafter, due to the solid skin layers thereon, the board is allowed to foam and expand only in the direction of the thickness so as to be formed of cells elongated in said direction, thereby providing high mechanical strength. With this patented process, however, the cells of the foamed board tend to be too much elongated in the direction of the thickness thereof. Therefore, provided with a hard skin on the upper and lower surfaces, a foamed board, if made thick, is likely to decrease in compressive strength.